Magneto-resistive device



J. M. HUNT MAGNETO-RESISTIVE DEVICE March 2, 1965 3 Sheets-Sheet 1 Filed June 13, 1960 INVEN TOR. JOHN M.HUNT

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March 2, 1965 N 3,172,032

MAGNETQ-RESISTIVE DEVICE Filed June 15. 1960 I 3 Sheets-Sheet 2 ERROR OUTPUT VOLTAGE I Emux Fig.4

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Attorney March 2, 1965 J. M. HUNT 3, 7

MAGNETO-RESISTIVE DEVICE Filed June. 11:5; 1960 v s Sheets-Sheet s l: SSQSEW T SOURCE H66 Y mmvm. JOHN M. HUNT Fig.7

Attorney United States Patent 3,172,632 MAGNETO-RESISTEVE DEVKCE John M. Hunt, Paio Alto, Caiifi, assignor to Generai Precision, Inc., Biughamton, N.Y., a corporation of Delaware Filed June 13, 1960, Ser. No. 35,829 11 Claims. (Cl. 323-66) This invention relates to electrical devices having impedance or resistance characteristics which may vary in the presence of magnetic fields, and more particularly to such magneto-resistive devices wherein the impedance may be controlled with sufiicient accuracy that the devices may be useful for performing analog computer functions such as multiplication and the like.

Certain materials exhibit a pronounced increase in electrical resistivity when placed in a magnetic field. This effect may be termed magneto-resistance and is most pronounced in bismuth and certain semi-conductors. One manifestation of this phenomenon results from the well-known Hall effect. In accordance with the Hall effect, a magnetic field may exert a force upon electrons or other electrical carriers flowing in a conductive medium and this force will be mutually perpendicular to the direction of the magnetic lines of fiux and to the direction of the current flow. Because of the magnetic forces, the electrons tend to accumulate on one side of the conductor, causing a transverse voltage to exist. If this transverse voltage (known as the Hall voltage) is shorted, the resulting transverse current flow in the magnetic field will cause a retarding potential, or back EM.F., which impedes the original current flow. The ultimate effect of the shorted Hall voltage is to cause an apparent increase in the resistance of the conductive medium upon the application of a magnetic field.

Certain materials such as bismuth, indium antimonide and indium arsenide exhibit magneto-resistance effects even though no specific provision is made to short the Hall voltage. Metallic bismuth has been found especially useful in magneto-resistance devices because it is easily workable and may be plated upon a backing material to form an electronically conductive path of a desired configuration, and because of bismuths inherently great magneto-resistance properties.

The possible use of magneto-resistance in electrical control circuits has been indicated by R. K. Willardson and A. C. Beer in an article entitled Magnetoresistance New Tool for Electrical Control Circuits published in the January 1956 issue of the Electrical Engineering magazine. Heretofore, the usefulness of magneto-resistive devices has been limited, because the variation in resistivity of a material such as bismuth when placed in a magnetic field cannot, under all conditions, be predicted to a sufficient degree of accuracy. The magnetic flux pattern of the field may be more intense in one area than in another and the resistivity variation is nonlinear with respect to the flux tensity variation; accordingly, it would be extremely diificult to calibrate such a device with any degree of accuracy. Since prior art magneto-resistive devices are inherently inaccurate, they cannot be tolerated in precision equipment such as analog computers.

An object of this invention is to provide an improved magneto-resistive device having a high degree of accuracy, and more specifically it is an object to provide an accurate magneto-resistive arrangement which is relatively simple and inexpensive to manufacture.

Another object of this invention is to provide an improved circuit for performing analog functions, and more specifically, it is an object to provide a method and means for performing multiplications, phase shifting and the like by the use of a magneto-resistive device.

3,172,032 Patented Mar. 2, 1965 ice A further object is to provide an improved magnetoresistive means for performing multiplications and the like, and to provide a method for accurate calibration of such means wherein the error is minimized by adjustments of fixed resistors associated with magneto resistors and of the scale factor or gain of an output amplifier.

Numerous other objects and advantages will be apparent throughout the progress of the specification which follows. The accompanying drawings illustrate certain selected embodiments of the invention and the views therein are as follows:

FIGURE 1 illustrates a magneto-resistive device constructed in accordance with the teachings of this invention wherein a grid of magneto-resistive material, plated or deposited upon a disc of supporting material, is placed between the poles of a magnet; 1

FIGURE 2 is a view of the disc having a pattern of magneto-resistive material plated thereon and adapted for positioning between the poles of a magnet;

FIGURE 3 is a schematic diagram illustrating an analog multiplying circuit using a magneto-resistive device in accordance with this invention;

FIGURE 4 is a graph illustrating the characteristics of the magneto-resistive device over a range of useful output voltage;

FIGURE 5 is another graph illustrating the possible output error from a multiplier over the range of useful output voltage;

FIGURE 6 is another circuit for performing multiplications wherein two similar magneto-resistive devices are used with a push-pull amplifier;

FIGURE 7 is a circuit for phase shifting a voltage using a magneto-resistive device; and

FIGURE 8 is a diagram illustrating the vector relationships of the circuit of FIGURE 7.

Briefiy stated, according to this invention, a magnetoresistive device includes a pair of nearly identical con: ductive paths of bismuth or other suitable magnetoresistance material which is formed by mechanical tech; niques, plating, depositing, or etching upon a suppQrting backing disc 13. The disc 13 is positioned between the poles 11 and 12 of an electro magnet, and a servo or feedback circuit is provided to control the exciting current of the magnet while sensing the resistance of one of the conductive paths. Although the two paths are electrically isolated from each other, the magneto-resistive characteristics are nearly identical, and therefore, the servo circuit controlling the resistance of one of the paths causes a substantially identical resistance in the other path. When used as a multiplier, a first analog signal applied to the servo circuit causes a desired resistance to appear in both electrical paths, and a second analog signal is coupled to and attenuated by the second electrical path to produce an output signal corresponding to the product of the two input signals. When used as a phase shifting circuit, a first of the resistive paths is coupled into a servo circuit while the second resistive path is serially coupled with a capacitor across a constant alternating voltage source.

FIGURE 1 illustrates the general arrangement of the magneto-resistive device in accordance with this invention. A magnet with the opposed spaced poles 11 and 12 provides a gap therebetween wherein a disc 13 is positioned. As indicated by dashed lines 14, a magnetic core is closed but for the gap between the poles 11 and 12. This magnetic core may assume various forms, as for example, the poles 11 and 12 may extend inwardly from an outer shell structure similar to that of a dynamic loudspeaker. Since the resistance variation .obtainable in this device corresponds to the range of flux density and the maximum flux obtainable, the Width or spacing of the gap 11-12 should be minimized. In a preferred form of this invention, the grid of magnetoresistive material is plated upon a thin wafer or disc 13 which is then clamped between the opposite poles 11' and 12 such that the ultimate gap may be of the order of .001 inch. It has been proposed that the effective gap be even further reduced by using a disc 13 of magnetic or ferrite material coated with an electrically insulating lacquer or other film. An obvious further extension would be to apply the insulating film directly to a poletface 12 and thence plate the magneto-resistive grid thereon.

FIGURE 2 shows a preferred arrangement for fabricating a disc 13 having a magneto-resistive grid. The disc 13 has been formed from a thin sheet of material, and bismuth has been electro-plated in the configuration as shown. A first pair of broadened areas 16 and 17 of bismuth plating are at the opposite ends of a first conductive path of the grid and constitute terminal points for connecting the bismuth resistor to further electrical circuitry. Similarly a second pair of broadened areas 18 and 19 constitute the terminals of a second respective path. The conductive paths are generally a plurality of parallel line segments 20, 21, 22, 23 etc. connected at opposite ends by segments which may be accurate and are portions of two concentric circles which generally outline the shape and dimensions of the magnetic pole pieces 11 and 12. Although FIGURE 2 shows but 12 line segments 20, 21, 22, 23 etc. it may be appreciated that this number may be increased. One grid was made and satisfactorily tested having 100 line segments arranged in 25 loops each loop having a pair of conductive paths extending from one side to the other and returning to the first side. The electrical paths between terminals 16 and 17 and between terminals 18 and 19 lie closely adjacent and parallel to each other throughout their entire length within the area of the magnetic field between the pole pieces 11 and 12. Indeed, it may be said that the conductive paths 16-17, and 18-19 lie on opposite sides of a nonconductive area extending in a zig zag manner through the magnetic field from a point 25 between the terminals 16 and 18 and a point 26 between the terminals 17 and 19. Since the two conductive paths 16-17 and 18-19 are similar in dimensions and configuration and extend through the same magnetic field, the magneto-resistive characteristics of one path is substantially the same as the magneto-resistive characteristics of the other. Although FIGURE 2 shows only two such paths, it may be appreciated that more than two substantially identical paths may be fabricated in a similar grid form. For example, the identical patterns of bismuth may be plated or otherwise deposited upon both sides of the supporting disc 13 such that the configuration of FIGURE 2 would supply four substantially identical conductive paths. As shown in FIGURE 1 a magneticfield producing means may comprise a magnetic core structure 11, 12 and 14 together with a winding 28 linking the core and having a pair of terminals 29 adapted to receive an excitation current. By providing a servo means (to be described in connection with FIGURE 3) for controlling the excitation current, and including one of the twin conductive paths of magneto-resistive material within a magnetic field, the resistance of both conductive paths may be controlled with extreme accuracy. Therefore, the structure of FIGURE 1 constitutes a controllable impedance device having an accuracy which is suitable for analog computations.

The usefulness of the magneto-resistive device of this invention may be further understood by considering the analog multiplier of FIGURE 3. A first analog input signal, X, may be negative in character and is applied to an operational amplifier 31 through a summing resistor 32. The amplifier 31 furnishes an output current which may flow through the winding 28 to generate a magnetic field designated by dashed lines 33 within which the twin magneto-resistive paths 34 and 35 extend between the respective terminals 16-17 and 18-19. A first of the magneto-resistive paths 34 is serially connected into and forms a part of a potential dividing network connected between a positive and negative potential source +E and E. The potential dividing network includes a resistor 36 and a balance resistor 37, the function of which will be described subsequently. This potential dividing network may furnish a voltage at a point 38 which corresponds to the resistance of the element 34 and to the intensity of the magnetic field 33. The voltage at the point 38 is fed back to the input of the amplifier 31 through a summing resistor 39.

In the operation of this invention, it is contemplated that the amplifier 31 may be an operational amplifier of conventional design and having a high forward or open loop gain. The negative feedback resulting from a variation in the resistance of the element 34 and returned to the input of the amplifier will be considerable and such that the feedback signal will substantially balance the input signal X. From the foregoing it may be appreciated that the amplifier 31 with the feedback arrangement constitutes a servo system whereby the resistance of the element 34 is determined by the input signal X. Since the elements 34 and are twins and substantially identical to each other, the resistance of the element 35 is likewise determined by the input signal -X through the servo system including the element 34.

The second magneto-resistive element 35 is coupled in series with a fixed resistor 40 which together constitutes a potential dividing network coupled to positive and negative voltages representative of the second analog input, l-Y and Y. The voltage appearing at the series connecting point 41 will therefore be a function of the first analog input X Which causes a variation in resistance, and the second analog input Y which is coupled to this re sistance. The voltage at the point 41 may be considered an analog output voltage representative of the product of X and Y, but it has been found advantageous to pass this voltage through another operational amplifier 42 to obtain the final XY analog output.

The accuracy of the variable resistance device and of the analog multiplier of this invention is illustrated in FIGURE 4, which is a graphical representation of the output error plotted against output voltage over the range from zero to the maximum output which may be passed by the amplifier 42. As may be seen from FIGURE 4 the error characteristic of the circuit of FIGURE 3 is complex and may not be described as a simple function. However, certain adjustments may be made in the circuit to effect an error reduction and improve the accuracy. Element 37 of FIGURE 3 is an adjustable resistor having a maximum resistance of a small percentage of the Zero field resistance of magneto-resistor 34. Introduction of a small series resistance as indicated by element 37 causes an output error which is a quadratic function of output voltage (that is, the resultant error is substantially represented by a constant off-set, a change in output scale factor and a term proportional to the square of output voltage). Accordingly, the element 37 will be adjusted to arbitrarily remove any observed quadratic component of output error.

A plot of output error versus output voltage after elimination of the quadratic error term is indicated in FIG- URE 4. It will be observed that the output error curve then consists of two error minima 45 and 47 and two error maxima 46 and 48 which may be bounded by a pair of diagonal parallel lines 43. The linear component of output error represented by the slope of the diagonal lines 43 which bound the error curve of FIGURE 4 may be interpreted as an error in output scale factor 50 and may be eliminated by suitable adjustment of the gain of amplifier 42. Similarly, the zero off-set (the error represented by the intercept of a median dotted line 43' of FIGURE 4 in the zero output voltage axis) may be eliminated by adjustment of series resistors 36 or 40 in FIG.-

URE 3. If both of the above adjustments are accomplished, the resultant error becomes a function of output as indicated in FIGURE 5. In general the peak error following such an adjustment procedure is substantially less than the error of the uncorrected elements. Improvements as great as one hundred to one having been obtained with experimental magneto-resistor pairs.

Magneto-resistive devices constructed in accordance With this invention have been tested and found to have an accuracy of .001%, or one part in 100,000. Thus, a multiplying circuit using this device may have equivalent accuracy, and is therefore superior to analog multipliers in present use. Indeed, the quality of the fixed resistors 37 and 40 imposes a more severe limitation upon the accuracy of the multiplier of FIGURE 3 than is inherent in the magneto-resistive device including the twin conductive paths 34 and 35, and the servo arrangement with the amplifier 31.

The accuracy of the variable resistance device of this invention depends on the fact that the two or more conductive paths are fabricated as nearly identical as possible. From an examination of FIGURE 2, it will become apparent that the greatest deviations or dissimilarities between the conductive path 16-17 and the path 18-19, lie in the top and the bottom of the configuration. If the magnetic field should have a region of greater or less intensity on one side, and if this non-uniformity of magnetic field should coincide with the top or the bottom of the grid pattern of FIGURE 2, a dissimilarity in magneto-resistive characteristic of the tWo conductive paths may result. To overcome this fault, the disc 13 may be rotated about a perpendicular axis through the center thereof. Thus, by rotatably positioning the disc 13 with respect to the magnetic field the balance between the two conductive paths may be optimized.

FIGURE 6 illustrates an alternative multiplier circuit using a push-pull amplifier 51 and a pair of twin magnetoresistive devices 52 and 53. .As in the circuit of FIGURE 3, a first analog input signal representative of a quantity X is applied with negative polarity to the amplifier 51 through a summing resistor 32'. The push-pull amplifier furnishes two output signals to the windings 54 and 55 which may be equal when the analog input X is zero, but which will become unbalanced for any positive or negative value of the input. The magneto-resistive elements of the two devices 52 and 53 are respectively connected in series to provide output voltages at the series junction points 38 and 41. As in FIGURE 3, the voltage at the point 38' is fed back to the input of the amplifier 51 through a summing resistor 39' and a servo arrangement is established such that the unbalancing effect in the values of the two resistors 56 and 57 is in accordance with the first input signalling X.

The voltage appearing at the point 41 results from the application of second input voltages +Y and Y and is further modified by the unbalanced resistive effect of the resistors 58 and 59. This voltage representative of the product XY may be passed into an operational amplifier 42 as in the embodiment of FIGURE 3.

FIGURE 7 shows an arrangement for using a magnetoresistive device as a phase shifting circuit. As in the case of FIGURES 3 and 6, an input signal X is applied to an operational amplifier 61 through a resistor 62 to provide an excitation current in the winding 63 of a magnetoresistive device 64. A first conductive element 65 of the device 64 is connected in series With a constant current source 66 for developing a feedback voltage at a point 67. The voltage at the point 67 is fed back to the input of the amplifier 61 through a resistor 68.

Thus, the amplifier 61 and feedback arrangement 68 provides a servo system for control of the resistive value of the element 65 and of the twin element 69. A transformer 70 supplies equal and opposite voltages E and E from a center tapped secondary winding to a resistancecapacitance circuit including the element 69 and a capaci- 6 tor 71. As the resistive value of the element 69 is varied, the voltage between ground and a point 72 remains constant in amplitude but shifts in phase.

The vector relationships of the alternating voltages of the circuit of FIGURE 7 are illustrated in the diagram, FIGURE 8. The transformer 70 provides the equal and opposite voltages E and E at its secondary winding, and the sum of the voltages across the variable resistance element 69 and the capacitor 71 is equal in value to E +E Since the voltage drop across the resistive element 69 is in phase with the current therethrough, and since the voltage across the capacitor 71 must lead the current by it follows that the voltage vector E (drop across the resistor 69) remains at an angle of 90 with the voltage E (across the capacitor 71). Therefore, a right triangle is formed by the voltage vector E the vector E and the combined vectors E and E The voltage at the output point 72 must lie on a semi-circular locus, as indicated by the vector B Therefore, the output voltage at point 72 must remain constant in amplitude and equal to E and E but may shift in phase as the resistance of the element 69 is varied.

As indicated heretofore, various other semi-conductive materials exhibit magneto-resistive eifects and could be useful as a variable resistance device. In addition to metallic bismuth, indium antimonide and indium arsenidc which exhibit natural magneto-resistive characteristics, further materials may be employed by providing transverse conductive bars at intervals along the conductive path for the purpose of short-circuiting the Hall voltage.

Metallic bismuth has proven superior to other known semi-conductive materials because of ease of fabrication and the fact that its effective resistivity may be doubled by the application of a magnetic field. In certain grid configurations similar to FIGURE 2, the resistance of the conductive paths in the absence of a magnetic field has been measured at 1,000 ohms, while the same conductive path assumes approximately 1,600 ohms in a magnetic field of approximately 16,000 gauss. The available change in resistivity of the bismuth element may be increased enormously by operation of the entire system at reduced temperature. For example, the resistivity of bismuth at liquid nitrogen temperature (approximately 192 C.) increases by a factor of greater than 50 to 1 in a field of 20,000 gauss. Accordingly, operation of the device at reduced temperature permits attainment of greater variation in resistance or reduction in the required field intensity to achieve a given range of resistance variation.

The variable resistance device may be used in an analog multiplier or in a phase shifting circuit as described above, but more broadly this device may be used in any circuit where a highly accurate controllable resistor is required. Obviously, further applications and further circuits may be found for a resistance element the value of which may be controlled with great accuracy and as a continuously variable function rather than a stepped function.

Changes may be made in the form, construction and arrangement of the parts without departing from the spirit of the invention or sacrificing any of its advantages, and the right is hereby reserved to make all such changes as fall fairly within the scope of the following claims.

The invention is claimed as follows:

1. A variable impedance device comprising a means for generating a magnetic field, at least two magneto-resistive elements positioned in the magnetic field, and a control means coupled to the magnetic field generating means, said control means being sensitive to variations in the impedance value of one of the magneto-resistive elements and being operable to control the intensity of the magnetic field.

2. A variable impedance device comprising a means for generating a magnetic field, a plurality of magnetoresistive elements positioned in the magnetic field, said magneto-resistive elements having similar dimensions and geometric configuration, and a control means coupled to the magnetic field generating means, said control means being sensitive to the impedance value of one of the magneto-resistive elements and being operable to control the intensity of the magnetic field.

3. A variable impedance device comprising a magnet having poles with a gap therebetween, a pluraltiy of magneto-resistive elements positioned in the gap, said magneto-resistive elements having similar dimensions and geometric configuration, and a control means associated with the magnet for inducing and controlling the intensity of magnetic flux in the gap, said control means being sensitive to the impedance value of one of the magneto-rcsistive elements.

4. A variable impedance device comprising a plurality of electrical paths of magneto-resistive material, said electrical paths having similar dimensions and configuration whereby the impedance values of the paths will be similar to each other when exposed to a magnetic field of a given intensity, and a means for generating a controlled magnetic field responsively coupled to one of the electrical paths.

5. A variable impedance device comprising a plurality of electrical paths of magneto-resistive material, said electrical paths being closely adjacent and parallel to each other and having similar dimensions and configuration whereby the impedance values of the paths will be substantially equal to each other when exposed to a magnetic field of a given intensity, and a means for generating a magnetic field responsively coupled to one of the electrical paths.

6. A variable impedance device comprising a grid of magneto-resistive material, said grid including a plurality of parallel straight line segments extending from one side thereof to the other, said line segments being joined at the ends to form at least two electrical paths of similar configuration and having similar magneto-resistive characteristics.

7. The variable impedance device in accordance with claim 6 wherein the magneto-resistive grid comprises bismuth plated upon a sheet of supporting material.

8. A variable impedance device comprising a supporting means adapted for positioning between poles of a magnet, a plurality of electrical paths of magneto-resistive material thereon, said electrical paths being closely adjacent and parallel to each other and having similar dimensions and a configuration corresponding to the magnetic poles, said electrical paths being operable to provide a plurality of impedances which vary with the intensity of the field of the magnet but which remain substantially equal to each other, and means for controlling the intensity of the magnetic field in response to impedance of one of the electrical paths.

9. A variable impedance device comprising a magnet having poles spaced apart with a gap therebetween, a disc having a magneto-resistive material thereon in a configuration forming a plurality of electrical paths, said electrical paths being closely adjacent and parallel to each '8 other and having similar configuration to provide impedances which are substantially equal to each other, said disc being positioned in the gap between the magnetic poles and being operable to provide equal variations in impedance in the plurality of electrical paths corresponding to variations in a magnetic field intensity.

10. A variable impedance device comprising a magnetic field generating means including opposed spaced magnetic poles and a grid of magneto-resistive material plated upon a backing sheet and positioned between the magnetic poles, said grid including a plurality of parallel straight line segments extending from one side thereof to the other across an area corresponding to the poles of the magnet, said line segments being joined at the ends thereof to form at least two electrical paths through the grid, said electrical paths being of similar configuration and repeatedly crossing and re-crossing through the magnetic field via the various line segments.

11. A variable impedance device comprising a magnetic field generating means including opposed spaced magnetic poles, and a grid of magneto-resistive material plated upon a backing sheet, said grid including a plurality of parallel straight line segments extending from one side thereof to the other across an area corresponding to the poles of the magnet, said line segments being joined at the ends thereof to form at least two electrical paths through the grid, said electrical paths being of similar configuration and repeatedly crossing and re-crossing through the magnetic field via the various line segments, said grid on the backing sheet being inserted between the magnetic poles and being rotatably positioned such that the magneto-resistive characteristics of one of the electrical paths is substantially equal to the magneto-resistive characteristics of the other.

References Cited by the Examiner UNITED STATES PATENTS 2,226,846 12/40 Clark 338-82 2,513,899 7/50 Taylor 235-494 2,529,117 11/50 Tompkins 333-29 2,536,806 1/51 .Hansen 32394 2,606,966 8/52 Pawley 33329 2,767,911 10/56 Hollingsworth 235-194 2,852,732 9/58 Weiss 32394 3,024,997 3/62 Sun 235194 3,024,998 3/62 Sun 235194 FOREIGN PATENTS 87,166 12/58 Netherlands.

OTHER REFERENCES Electronic Engineering, vol. 30, November 1958, C. Hilsum Multiplication by Semi Conductors, pp. 664 666, published in London.

MILTON O. HIRSHFIELD, Primary Examiner. CORNELIUS D. ANGEL, Examiner. 

1. A VARIABLE IMPEDANCE DEVICE COMPRISING A MEANS FOR GENERATING A MAGNETIC FIELD, AT LEAST TWO MAGNETO-RESISTIVE ELEMENTS POSITIONED IN THE MAGNETIC FIELD, AND A CONTROL MEANS COUPLED TO THE MAGNETIC FIELD GENERATING MEANS, SAID CONTROL MEANS BEING SENSITIVE TO VARIATIONS IN THE IMPEDANCE VALUE OF ONE OF THE MAGNETIC-RESISTIVE ELEMENTS AND BEING OPERABLE TO CONTROL THE INTENSITY OF THE MAGNETIC FIELD. 